High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Variable geometry turbochargers (VGT) allow the intake airflow to be optimized over a range of engine speeds. This is accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
EGR systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated exhaust gas displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture.
Both the VGT and EGR regulate gas flow from the exhaust manifold, and their effect is, therefore, coupled through the conditions in the exhaust manifold. Excessive EGR rates displace the intake of fresh air and may lead to incomplete combustion of the injected fuel which, in turn, could cause visible levels of smoke and increased levels of emissions. Moreover, this could negatively affect fuel economy and/or performance. Thus, for effective control of diesel engines with EGR systems, it is necessary to control the EGR flow precisely, not only in steady state but also in transient conditions. In the case of diesel engines equipped with a VGT, the actual flow through the EGR valve can vary greatly, even for a fixed EGR valve opening, depending upon the exhaust pressure fluctuations generated by opening or closing the inlet guide vanes of the VGT. In such a case, it is difficult to control the EGR flow based solely on EGR valve position.
Accordingly, current engine designs utilize a mass airflow (MAF) sensor and a manifold absolute pressure (MAP) sensor for proper regulation of airflow into the engine and, consequently, EGR flow in VGT-equipped engines. Regulation of airflow is important because it directly relates to the amount of fuel that can be injected to meet driver demand, while avoiding visible smoke and excessive particulate emissions.
Set points for MAP and MAF are developed by engine mapping which is referenced to fuel demand as well as and engine speed. The EGR valve is used to control MAF, and the VGT is used to control MAP. Neither EGR flow nor VGT position is typically measured. Conventional MAF sensors, however, have limited accuracy and are more expensive than MAP sensors or exhaust manifold pressure (EXMP) sensors.